Summary

Abstract

The establishment of cortical circuitry was studied by analyzing the spatial distribution of synapses in somesthetic cortex (SI) of neonatal dog. Relevant principles of cortical development are critically evaluated in a selective review of early and recent literature. A quantitative electron microscopic method was devised in order to analyze synaptic locations with respect to cytoarchitectonic layers of the cortex. It was demonstrated that synapses are not uniformly distributed through the cortex: strata of high synaptic density (synaptic strata) alternate with strata of low synaptic density. The synaptic strata have a definite relationship to cytoarchitectonic layers. Moreover, deep and superficial synapse-dense strata coincide with layers of precocious dendrite maturation. The synaptic strata are used to explain the polarity of evoked potentials characteristic of neonatal cortex. Both physiologic and morphologic data lead us to postulate that, in fetal life, the earliest synapses are established deep in the cortex. The functional significance of “synaptoarchitectonic” layers is compared with that of “cytoarchitectonic” layers. Principles relevant to a general formulation of developing cortical circuitry are discussed.